Embodiments of the invention relate generally to high intensity focused ultrasound (HIFU), and more specifically, to a system and method of employing magnetic resonance—acoustic radiation force impulse (MR-ARFI) imaging feedback for fast and robust focusing of the HIFU.
Focused ultrasound therapy involves delivering ultrasound energy to localized regions of tissue from externally (non-invasive) or internally (minimally-invasive) located transducers. The amount of ultrasound energy delivered to tissue dictates the nature of the biologic effect produced at that location. At high intensities with continuous exposure, ultrasound energy can generate enough heat to cause irreversible thermal damage through coagulation. As the exposure is reduced in duty cycle to short pulses, the mechanical energy associated with ultrasound dominates and can be used to generate a range of bio-effects, including: vascular occlusion or hemorrhage, permeation of cells, and tissue-homogenization.
For this purpose, a piezo-ceramic transducer is placed externally to the patient, but in close proximity to the tissue to be ablated (the “target”). The transducer converts an electronic drive signal into mechanical vibrations, resulting in the emission of acoustic waves (a process hereinafter referred to as “sonication”). The transducer may be shaped so that the waves converge in a focal zone. Alternatively or additionally, the transducer may be formed of a plurality of individually driven transducer elements whose phases (and, optionally, amplitudes) can each be controlled independently from one another and, thus, can be set so as to result in constructive interference of the individual acoustic waves in the focal zone. Such a “phased-array” transducer facilitates steering the focal zone to different locations by adjusting the relative phases between the transducers, and generally provides the higher a focus quality and resolution, the greater the number of transducer elements. Magnetic resonance imaging (MRI) may be utilized to visualize the focus and target in order to guide the ultrasound beam.
The relative phases at which the transducer elements need to be driven to result in a focus at the target location depend on the relative location and orientation of the transducer surface and the target, as well as on the dimensions and acoustic material properties (e.g., sound velocities) of the tissue or tissues between them (i.e., the “target tissue”). Thus, to the extent the geometry and acoustic material properties are known, the relative phases (and, optionally, amplitudes) can be calculated. In practice, however, knowledge of these parameters is often too incomplete or imprecise to enable high-quality focusing based on computations of the relative phases alone. For example, when ultrasound is focused into the brain to treat a tumor, the skull in the acoustic path may cause aberrations that are not readily ascertainable. In such situations, treatment is typically preceded by an auto-focusing procedure in which, iteratively, an ultrasound focus is generated at or near the target, the quality of the focus is measured (using, e.g., thermal imaging or acoustic radiation force impulse (ARFI) imaging), and experimental feedback is used to adjust the phases of the transducer elements to achieve sufficient focus quality.
The number of sonications in such an auto-focusing procedure is typically at least three times the number of individually controlled transducer elements, and even more sonications may be needed to overcome measurement noise. For example, for a transducer array with 1,000 elements, auto-focusing typically involves a systematic series of 3,000 or more sonications to optimize the focus thereof. The auto-focusing procedure may thus take a substantial amount of time, which may render it impracticable or, at the least, inconvenient for a patient. Further, during the auto-focusing sonications, ultrasound energy is inevitably deposited into the tissue at and surrounding the target, potentially damaging healthy tissue.
Attempts have previously been made to improve the auto-focusing procedure using MR-ARFI to measure the quality of the focus, with such MR-ARFI techniques being employed both to reduce the time required for performing the procedure and to minimize the effect of pre-therapeutic sonications. For example, a previous attempt for autofocusing using MR-ARFI is described in “Ultrasound focusing using magnetic resonance acoustic radiation force imaging: Application to ultrasound transcranial therapy” to Y. Hertzberg et al., Med Phys, Vol. 37, No. 6, 2010, in which the array of ultrasound transducers was divided into n groups (i.e., sub-arrays)—with it being assumed that all transducers in each group are approximately the same. The phase/amplitude of each group of transducers was then randomly changed until a reasonable focus was achieved. The method described by Hertzberg et al., however, is still time consuming and is less systematic than is desired. That is, the method described in Hertzberg et al. disregards MR-ARFI measurements at all voxels other than the center of the focus, such that a large number of image acquisitions are still required.
It would therefore be desirable to provide a system and method for MR-ARFI-based autofocusing of a phased array of transducer element to create a high-quality ultrasound focus. It would also be desirable for such a system and method to reduce the number of image acquisitions required to perform the autofocusing so as to reduce the time required for performing the autofocusing procedure and to minimize the effect of pre-therapeutic sonications, thereby making the MR-ARFI-based autofocusing clinically-feasible, and possible in near-real time.